How do you calculate a boiler efficiency gap?

Subtract measured efficiency from target efficiency to get the absolute gap, then divide that gap by the target. With an 84% target and 78% measured, the absolute gap is 6 points and the percentage gap is 6 / 84 = 7.14%.

What is a good boiler efficiency gap?

A gap under about 2 points is normal drift between tune-ups. A 6-point gap like the example (7.14% of target) signals real losses — often high excess air, fouled tubes, or elevated stack temperature worth investigating.

Why is my measured efficiency lower than the target?

The most common causes are excess combustion air (too much O2 in the flue), high stack temperature from soot or scale fouling, unmanaged blowdown, and burner drift since the last tune. Each adds to the gap.

Is efficiency gap the same as fuel savings?

Not directly, but it is the driver. Closing a 6-point gap roughly reduces fuel input by that fraction of the efficiency ratio, so a boiler burning $500k of fuel a year could recover a meaningful share by returning to its 84% target.

Should I use combustion efficiency or fuel-to-steam efficiency?

Be consistent. Combustion efficiency ignores blowdown, radiation, and jacket losses, so it reads higher than fuel-to-steam. Enter target and measured on the same basis or the gap is misleading.

What target efficiency should I enter?

Use the manufacturer's nameplate rating or your ASME performance-test baseline — commonly 80-85% HHV for firetube gas boilers and higher for condensing or economizer-equipped units.

Plant Utilities calculator

Boiler Efficiency Gap Calculator

The boiler efficiency gap measures how far a steam or hot-water boiler falls short of its design or benchmark combustion efficiency. Plant utility engineers and energy managers use it after a combustion tune-up, flue-gas analysis, or annual efficiency test to quantify wasted fuel. A gap of just a few points on a large boiler can mean tens of thousands of dollars in extra natural gas or fuel oil per year. It turns a raw efficiency reading into a prioritized action item for the boiler room.

What this calculator does

Compare measured boiler efficiency to the target so teams can see the efficiency gap that may justify tuning or maintenance.
Use it when reviewing boiler efficiency gap for a utility budget, maintenance priority, capacity check, energy project, or production support plan.
It computes the absolute point difference between your target and measured boiler efficiency, then expresses that difference as a percentage of the target.

Formula used

Efficiency gap = target boiler efficiency - measured boiler efficiency
Gap percentage = efficiency gap ÷ target boiler efficiency

Inputs explained

Target boiler efficiency:
Measured boiler efficiency:
Reference boiler efficiency:

How to use the result

Use it right after a flue-gas analysis, seasonal combustion tune, or when comparing a boiler's current stack loss against its nameplate or ASME PTC 4 design efficiency.
It only compares two efficiency numbers you supply; it does not calculate combustion efficiency itself from O2, CO2, or stack temperature, and it assumes both figures use the same basis (e.g. both higher heating value).

Current U.S. benchmarks

Industrial electricity averages 8.66 cents per kWh across the U.S. (EIA, Apr 2026), up 5.5% from a year earlier. Energy-intensive steps carry this directly into unit cost.

Common questions

How do you calculate a boiler efficiency gap? Subtract measured efficiency from target efficiency to get the absolute gap, then divide that gap by the target. With an 84% target and 78% measured, the absolute gap is 6 points and the percentage gap is 6 / 84 = 7.14%.
What is a good boiler efficiency gap? A gap under about 2 points is normal drift between tune-ups. A 6-point gap like the example (7.14% of target) signals real losses — often high excess air, fouled tubes, or elevated stack temperature worth investigating.
Why is my measured efficiency lower than the target? The most common causes are excess combustion air (too much O2 in the flue), high stack temperature from soot or scale fouling, unmanaged blowdown, and burner drift since the last tune. Each adds to the gap.
Is efficiency gap the same as fuel savings? Not directly, but it is the driver. Closing a 6-point gap roughly reduces fuel input by that fraction of the efficiency ratio, so a boiler burning $500k of fuel a year could recover a meaningful share by returning to its 84% target.
Should I use combustion efficiency or fuel-to-steam efficiency? Be consistent. Combustion efficiency ignores blowdown, radiation, and jacket losses, so it reads higher than fuel-to-steam. Enter target and measured on the same basis or the gap is misleading.

Last reviewed 2026-05-12.